Currently if there is uncancellable iproto request Tarantool shutdown
will hang. Let's instead give it some time and then panic.

On this way it is good to make iproto_drop_connections() fail on timeout.
It is used in `box.ctl.iproto_lockdown` which is better to fail on
timeout than to hang indefinitely too.

In Tarantool CI which is run with TEST_BUILD set, we set the timeout to
the infinity. This is on par with current fiber_shutdown() behaviour.
We will not change the latter for a while because there is already
several tests that count on that. Also it is currently easier to test
that there is no hang than to test exit status.

Part of #8423

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